Transition metal organometallic compounds & Catalysis

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Transition metal organometallic compounds & Catalysis. French ChemistL. C. Cadet1760As 2 Me 4 dicacodyl. Which one is organometallic? Ni(CO) 4 or NaCN ?. Metal-carbon bond: a few from many?. Organometallic Compound: Looking closer. 18 en rule. 1920British ChemistSidgwick. - PowerPoint PPT Presentation

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<ul><li><p>Transition metal organometallic compounds &amp; CatalysisMetal-carbon bond: a few from many?Which one is organometallic? Ni(CO)4 or NaCN ?French ChemistL. C. Cadet1760As2Me4 dicacodyl</p></li><li><p>Organometallic Compound: Looking closer</p><p>Ligand Name Bonding Type Molecular Hydrogen: H2 Hydride H- M-H </p><p>Phosphine: PR3M-PR3 Carbonyl: CO Alkyl , ArylM-CR M-PhAlkene</p></li><li><p>18 en ruleOrganic compounds Octet ruleOrganometallic 18 electron rule* 18 valance electron inert gas configuration 1920British ChemistSidgwick</p></li><li><p>To determine the electron count for a metal complex:Determine the oxidation state of the transition metal center(s) and the metal centers resulting d-electron count. To do this one must:a) note any overall charge on the metal complexb) know the charges of the ligands bound to the metal centerc) know the number of electrons being donated to the metal center from each ligand 2)Add up the electron counts for the metal center and ligandsCounting the number of electrons</p></li><li><p>Counting the number of electrons</p><p>Ligand Name Bonding Type Formal Charge Electrons donated Molecular Hydrogen: H2 02Hydride H- M-H -12Halide X-M-X-12Amine, phosphine, arsine: NR3, PR3, AsR3 M-NR3 M-PR3 02Carbonyl: CO 02Alkyl , ArylM-CR M-Ph-12Alkene</p><p>-12</p></li><li><p>CatalysisHeterogeneousHomogeneousA catalyst lowers the activation barrier for a transformation, by introducing a new reaction pathway.It does not change the thermodynamics!!</p></li><li><p>Synthesis of chemicals pharmaceutical, agriculturalCatalytic converter environmentalBiological system efficient catalystCatalysis : Why?Organometallic compounds, metals etc.</p></li><li><p>How to select an efficient catalyst?Activity: related to rate of reaction (also called turnover)efficient catalyst: good activityTurnover frequency (N) N = /[Q]Large turnover frequency efficient catalystSelectivity: Byproducts should be minimizedLifetime: It is costly to replace the catalyst frequentlyCost: The acceptable cost depends upon the catalyst lifetime, product value lifetime and product valuePoisoning: decomposition of catalyst, adsorption of reactant/product</p></li><li><p>Coordination compounds in catalysis Nobel Prizes Yves Chauvin,Robert H. Grubbs and Richard R. Schrock.2001KNOWLES, NOYORI, SHARPLESS1973WILKINSON1963ZIEGLER, NATTA1918HABER1909OSTWALD</p></li><li><p>Hydrogenation of Unsaturated HydrocarbonsThe most common catalystWilkinsons Catalyst, [RhCl(PPh3)3]NOBEL : 2001</p></li><li><p>Wilkinsons Catalyst (WC)Chlorotris(triphenylphosphine)rhodium(I)square planar d8 configuration</p></li><li><p>Geoffrey Wilkinson Born July 14, 1921, Yorkshire, England Ph.D from Cal Berkeley studying with Glenn Seaborg First published compound in 1965 in Journal of the Chemical Society - Chemical CommunicationsNobel Prize in Chemistry 1973 (shared with Ernst Otto Fischer) for their pioneering work, performed independently, on the chemistry of the organometallic, so called sandwich compounds.Organometallic compounds prepared by Wilkinson in display at Harvard Univ.</p></li><li><p>Synthesis of WCCommercially available </p></li><li><p>Catalytic steps(a) Ligand coordination and dissociationFacile coordination of the reactant and facile loss of products.Coordinatively unsaturated - 16-electron complexes(b) Oxidative additionMetal must possess a non-bonding electron pairCoordinatively unsaturatedOxidation of metal by two units Mn to Mn+2-occurs when a complex behaves simultaneously as a Lewis base and a Lewis acid</p></li><li><p>Oxidative addition</p></li><li><p>(c) Insertion or migrationMigration of alkyl and hydride ligands</p></li><li><p>(d) Nucleophilic attack</p></li><li><p>(d) Reductive eliminationInvolves decrease in the oxidation and coordination number</p></li><li><p>Hydrogenation of Unsaturated HydrocarbonsG0 = -101 kJ/mol</p></li><li><p>(1) Oxidative addition(2) Ligand DissociationWC in alkene Hydrogenation: Catalytic Steps</p></li><li><p>(3) Ligand Association(4) Migration/InsertionWC in alkene Hydrogenation: Catalytic Steps</p></li><li><p>(5) Ligand associationWC in alkene Hydrogenation: Catalytic Steps</p></li><li><p>(6) Reductive eliminationWC in alkene Hydrogenation: Catalytic Steps</p></li><li><p>WCINACTION</p></li><li><p>Rate of the reaction decreases as the alkyl substitution increasesHighly sensitive to the nature of the phosphine ligandAnalogous complexes with alkylphosphine ligands are inactiveHighly selective for C=C over C=OWC in alkene Hydrogenation: Additional Notes* Laboratory scale organic synthesis* Production of fine chemicalsApplications</p></li><li><p>Chiral phosphine ligands have been developed to synthesize optically active products.Synthesis of L-DOPA (Used in the treatment of Parkinsons diseases)Synthetic route was developed by Knowles &amp; co-workers at Monsanto Dr. William S. Knowles received Nobel prize in chemistry 2001 along with other two scientists.Alkene Hydrogenation &amp; Chirality &amp; Nobel</p></li><li><p>Alkene Hydrogenation, Chirality &amp; Nobel</p></li><li><p>Non-superimposable mirror imageEnantiomeric excess = (moles of major enantiomer - moles of other enantiomer / Total moles of both enantiomers) 100 </p></li><li><p>phenylanisylmethylphosphine (PAMP)Dimeric product is DiPAMP</p></li><li><p>Additional notes For interested students(a) Ligand coordination and dissociationFacile coordination of the reactant and facile loss of products.Coordinatively unsaturated - 16-electron complexes(b) Oxidative additionMetal must possess a non-bonding electron pairCoordinatively unsaturatedOxidation of metal by two units Mn to Mn+2-occurs when a complex behaves simultaneously as a Lewis base and a Lewis acid</p></li><li><p>Oxidative addition</p></li><li><p>(c) Insertion or migrationMigration of alkyl and hydride ligands</p></li><li><p>(d) Nucleophilic attack</p></li><li><p>(e) Reductive eliminationInvolves decrease in the oxidation and coordination number</p></li></ul>

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